/Engine/Source/Runtime/Foliage/Public/FoliageStatistics.h
return number of foliage instances with their mesh set to Mesh that overlap the sphere
static int32 FoliageOverlappingSphereCount ( [UObject](API\Runtime\CoreUObject\UObject\UObject) * WorldContextObject, const [UStaticMesh](API\Runtime\Engine\Engine\UStaticMesh) * StaticMesh, FVector CenterPosition, float Radius )